Fish are affected by temperatures in a similar way as reptiles, but it's important to think about certain properties of their environments that make it a bit simpler for fish. First, fish metabolism and functioning is very much affected by temperature. If you have ever had pet fish, you were definitely not allowed to keep tropical fish with goldfish due to different temperature requirements. Fish generally stay in one habitat of the ocean, where they are adapted to the temperature, rather than swimming the entire ocean freely. Fish adapted to colder environments, like salmon, wouldn't be found in the tropics. Now, why do fish not show the same sluggish behavior that reptiles do in the cold or in the winter? This is because water maintains its temperature much better than air (if you've had Chemistry, the heat per volume required to raise the temperature of air is much less than the heat per volume required to raise the temperature of water). If you're ever trying to boil water, it's pretty clear that air heats up much faster than water. So, the water is naturally resistant to temperature changes in the air, and there isn't much daily variation in water temperature. Thus, fish maintain their internal temperatures by staying in water of the correct temperature, which is much less sensitive to day and night than the air.

As a bonus interesting fact, it is thought that warm-bloodedness evolved to allow our distant, distant ancestors (the first mammals, tens to hundreds of millions of years ago) to find food and be active at night, so they could avoid the large reptile predators of the day. The reptiles couldn't hunt at night due to cold-bloodedness, so the mammals were safe being nocturnal. When the asteroid hit wiping out the dinosaurs, warm-blooded animals were able to survive the temperature change, while large reptiles couldn't.

This is a very cool question. Indeed, fish are ectotherms, which means their body temperatures are controlled by their environment. It looks like different kinds of fish deal with the problem of cold water in a variety of ways. For instance, in some fish, capillary blood flows in the opposite direction to the water flowing over their gills, which allows them to have countercurrent exchange, thereby maximizing the amount of heat transfer from the water to the fish.

Another example are fish that live at the surface of the Antarctic Ocean. These fish have a kind of "anti-freeze" consisting of glycoproteins in their blood, as a defense against touching ice, which could induce a nucleation point from which ice would spread through their bodies. You can read more about it here: cold animals

In that article you can also find a discussion of the fish that live way below the surface of the water, that instead of having anti-freeze, simply live in a "super-cooled" state. This means that even though the freezing point of their bodies is higher than the surrounding water, their blood doesn't become solid but stays liquid. As an aside: if you would like to learn more about supercooling, there are some interesting youtube videos I'm sure you could find about it!

Furthermore, I think the problem your question implies -- that of a fish becoming very torpid in colder water -- is most likely a problem for fish that found themselves out of their usual environment. For instance, if you were to take a tropical fish and put it in Antarctic water for which it is not acclimated, much less evolved, then that poor fish would probably freeze. However, I suspect that fish that are living in their natural habitats have found ways to deal with the "relatively" cold water surrounding them.

Good question; I have often wondered this. The answer is that it depends on the rates of chemical reactions that the animal needs in order to do its biochemistry. However, fish do need warmth in order to be really active, and the most active fish (like tuna) are in fact warm-blooded, at least partially. The fact is that water feels to you as though it were cold because it conducts heat away from you very fast, while air does less so because it is less dense, even when the air and water are the same temperature.